Device states occur in the global system G0 working state. They were defined to enable device vendors to design ACPI-compliant products, so that operating system that support ACPI, like Windows XP, can manage the devices. There are four D-states, but vendors can choose not to implement all states.

D0 State

In this state, the device is operating at its full power and full functionality.

Example : A DVD-ROM drive in active use.

D1 State

The device can choose to discard its context.

However, the bus connected to this device should not do anything to cause the context loss in the device.

Power consumption is lower than the D0 state, as some working units in the device will shut down.

Example : After idling for some time, the laser in the DVD-ROM drive will automatically turn off, but the drive controller will still be active.

D2 State

It is similar to D1, but the bus is free to some power management, like lower the current and voltage.

This can save more power, but it will take a longer time to wake up from the D2 state.